In this supplemental request to Grant Application No. 20686, the objectives of this work are the isolation and identification of serotonergic receptors from parts of the midbrain and the interaction of such receptors with a variety of psychotropic agents ranging from LSD and its derivatives (i.e., substances incorporating the indole nucleus) to morphine and its analogues. At the time of completion of this request serotonergic receptors have been separated from other aminergic structures of the end (synaptic membranes), including special receptor sites for tryptamine, which proved to be different from those for serotonin. The latter appear to be reacting with psychotropic agents of the family of LSD. Further progress in the understanding of the nature and the mechanism of action of psychotropic drugs depends on: a) kinetic work; b) methodology permitting fast determination of radioactivity; c) identification of specific groups present at the receptor sites; and, d) structural studies on the isolated receptors. We hope that the above needs will be fulfilled with the additional instruments as requested herein.